Conventionally, a transparent media detection sensor for detecting transparent media (media having optical transparency including translucent media) has been known widely, and for example, in searching the Internet using “transparent media detection sensor” as a keyword, it is known that many sensors are commercially available.
As the transparent media detection sensor, there are various types such as a type using a reflector (regression reflector), for example, see Patent Document 1, a type of using a polarizer, for example, see Patent Document 2, and types of using a CCD sensor or line sensor, for example, see Patent Document 3. While such transparent media detection sensors have high accuracy, the sensors are susceptible to improvements in the cost.
In addition, in the field of business equipment, in consideration of the cost and requirement accuracy, sensors having a light emitting device (LED) and light receiving device (phototransistor) have been widely used. For example, Patent Document 4 discloses techniques for detecting a center portion of a printing area (label) temporarily attached to a continuous medium (label continuous medium) using a transmission sensor.
In this type of sensor, a sensor control circuit as shown in FIG. 12 is generally used. In other words, the anode side of a light emitting device Le (LED) is connected to Vcc (for example, +3.3 V), and the cathode side is connected to a collector of a transistor Tr1. The base of the transistor Tr1 is connected to an output terminal of an operational amplifier (OP) via a resistance R1, and a positive-phase input terminal of the operational amplifier (OP) is connected to a DA output port of a microprocessor MP via a DA converter (DA). Further, an emitter of the transistor Tr1 is connected to one end of a resistance R2 connected to a negative-phase input terminal of the operational amplifier (OP) and at the other end to a ground (hereinafter, abbreviated as GND). Therefore, by outputting the digital voltage from the microprocessor MP, it is possible to cause the light emitting device (Le) to emit light.
On the other hand, a collector of the phototransistor constituting the light receiving device Lr is connected to Vcc via a resistance R3, and an emitter is connected to GND. Further, the collector of the phototransistor is connected to an AD input port of the microprocessor MP with an AD converter incorporated. Therefore, the microprocessor MP is capable of capturing the output voltage of the collector of the phototransistor i.e. the output voltage output from the sensor control circuit (sensor Se).
In addition, as techniques related to the present invention, the inventor of the present invention proposed a printer capable of suppressing fluctuations in front end detection of media (see Patent Document 5).